It is known that on heating, water-on-demand can be more efficient than heating and storing a tank of water as in a domestic hot water tank. To date most on-demand hot water heaters are powered by either electricity or gas (natural gas or propane). It is also known that heat exchangers have been used in industrial applications where high pressure steam is used to provide on-demand heating of water using primarily large U-tube or straight-through tube-type heat exchangers as in the Flo-Rite Steam Instantaneous Water heater made by Armstrong of Three Rivers, Mich., and the Instantrol Steam/Fluid Heater Package made by Robertshaw of Knoxville, Tenn. The heated water temperature is generally maintained by a thermally responsive valve that controls the inlet steam.
Another on-demand water heater is the MicroMix II produced by the Graham of Batavia, N.Y., in which water heated in a steam fed helical-coil heat exchanger is mixed with cold feed water using a thermally controlled mixing valve to achieve the desired output water temperature.
Applicant is also aware of patents regarding such on-demand heaters and variants such as:
U.S. Pat. No. 6,739,288 which issued to Kumamoto on May 25, 2004, teaches a method of regulating temperature between a heat exchanger and a condensate recovering unit by means of a steam ejector valve.
U.S. Pat. No. 4,278,069 which issued to Clark, Jr. on Jul. 14, 1981, teaches a water heater using an elongated vessel having a heat exchanger at one end leaving a volume storage and blending zone at the other end of the vessel.
U.S. Pat. No. 5,233,970 which issued to Harris on Aug. 10, 1993, teaches a helical coil heat exchanger in between two cylindrical metal sheets immersed in a tank of water to be heated.
U.S. Pat. No. 6,275,655 which issued to Rixen on Aug. 14, 2001, teaches a supplemental heating system configured for recreational vehicle use where hot engine coolant is circulated through a heat exchanger to heat potable domestic water using a compact heat exchanger and in some cases either thermal control of the feed valve, or mixing of hot and cold water to achieve desired water temperatures.
Many light industrial or commercial applications such as dry cleaners, hotels, etc presently use boilers for supply of steam and hot water. Typically the boilers will be run continuously during business hours, if not longer. In the use of boilers producing hot water, once the water is heated it may be stored for later use. In the case of boilers producing steam, the steam may not be usefully stored as it will return to condensate. Consequently, in the case of steam producing boilers, herein referred to as steam boilers, there exists a thermal inefficiency in producing steam which goes un-used and so is lost to condensate as it cools, only to be returned to the boiler to be re-heated. One object of the present invention is, in one embodiment, to recapture such wasted heat in an on-demand water heater by the use of a steam-to-water heat exchanger. Another object is to further recapture waste heat from the steam boiler by using the return steam condensate to pre-heat cold water entering the steam-to-water heat exchanger.